An electrical power steering system (EPS) assists a vehicle driver's steering operation by driving force of a motor. The EPS disclosed in, for example, JP2003-182598A, performs failure diagnosis to diagnose presence or absence of a motor and a motor driving circuit during a period from turn-on of an ignition switch to a start of assisting steering operation.
In JP2003-182598A, a motor relay provided between the motor driving circuit and the motor during the failure diagnosis is performed. As a result, when a driver operates a steering wheel, the motor operatively coupled to the steering wheel through a gear responsively generates an induction voltage, which will likely lower accuracy of failure diagnosis. It is therefore proposed, in the failure diagnosis after turn-on of a motor relay, to monitor an angular velocity of the motor so that the failure diagnosis is postponed and performed when the angular velocity is lowered to a predetermine value. If the failure diagnosis is postponed and performed only after the induction voltage becomes less than a predetermined value, a period from the turn-on of an ignition switch to a start of assist operation of the EPS becomes long.